1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly relates to an image forming apparatus which conveys a sheet onto which a toner image has been transferred while causing the sheet to form a loop in a region between a transfer unit and a fixing unit.
2. Description of the Related Art
In a conventional electro-photographic type image forming apparatus, after a toner image formed on an image bearing member is transferred onto a sheet serving as a transfer material by a transfer unit, the toner image is fixed on the sheet by introducing the sheet to a fixing unit and heated thereby. In this case, because the sheet is conveyed while carrying the unfixed toner image, if conveyance of the sheet becomes unstable, a printed surface thereof that carries the unfixed toner image may contact members within the image forming apparatus, and thus the toner image may be damaged to cause a defective image. Further, if a non-printed surface which does not carry the unfixed toner image is scraped against the members within the image forming apparatus, the sheet may be electrically charged to cause the toner image to be damaged, and thus this may result in a defective image to be generated. Furthermore, paper creases may be generated if behavior of the sheet in a conveyance period becomes unstable. Accordingly, it is necessary to stably convey the sheet from the transfer unit to the fixing unit.
Therefore, in the conventional image forming apparatus discussed in Japanese Patent Application Laid-Open No. 07-234604, for example, a loop detection sensor for detecting a loop of the sheet is disposed on a conveyance guide arranged between a fixing unit and a transfer unit, and in order to convey the sheet stably, conveyance speed of the fixing unit is controlled to cause the amount of loop formed on the sheet to be kept within a predetermined range.
However, in the conventional image forming apparatus, there may be a case where the sheet is conveyed from the transfer unit to the fixing unit while warping in a width direction orthogonal to the sheet conveyance direction. In such a case, the sheet will loop while warping in the width direction. Hereinafter, the above-described loop is referred to as “lopsided loop”. If the sheet loops lopsidedly as described above, an amount of the loop becomes different at both end portions in the width direction of the sheet. Therefore, it is difficult to appropriately control the loop amount when loop control is executed.
In a case where the loop amount cannot be controlled appropriately, the loop amount will be excessively increased on one side in the width direction to cause a non-printed surface of the sheet to be strongly scraped against the conveyance guide, or conversely, the loop amount will be excessively decreased on one side in the width direction to cause a printed surface of the sheet to contact with members within the image forming apparatus. As described above, if the loop control cannot be executed stably, a problem such as defective images or creases may be generated caused by conveyance failure of the sheet in a region between the transfer unit and the fixing unit.